Legen undersøker lungene.

Because legen is the definite singular form of lege (doctor).

• en lege = a doctor
• legen = the doctor

In Norwegian, the definite article is usually added to the end of the noun as a suffix, instead of being a separate word like the in English.

Because lungene is the definite plural form of lunge (lung).

Here is the full pattern:

• ei/en lunge = a lung
• lunger = lungs
• lungene = the lungs

So Legen undersøker lungene means The doctor examines the lungs, not just lungs in a general sense.

That is a normal feature of Norwegian grammar. In many cases, definiteness is shown with an ending attached to the noun.

Examples:

• bil = car
• bilen = the car
• hus = house
• huset = the house

This is very different from English, where the comes before the noun.

Undersøker is the present tense of the verb å undersøke (to examine / to investigate).

Examples:

• å undersøke = to examine
• undersøker = examines / is examining

Norwegian present tense is often formed by adding -r to the infinitive stem:

• snakke → snakker
• jobbe → jobber
• undersøke → undersøker

It can mean both, depending on context.

Norwegian present tense often covers what English expresses as either:

• examines
• is examining

So Legen undersøker lungene could mean:

• The doctor examines the lungs
• The doctor is examining the lungs

Usually the context tells you which is most natural.

This is the normal basic Norwegian sentence order:

Subject + Verb + Object

So:

• Legen = subject
• undersøker = verb
• lungene = object

This is very similar to English:

• The doctor
  • examines
    • the lungs

Yes. Lungene is the direct object — the thing being examined.

You can find the object by asking:

• What does the doctor examine?
• The lungs

So:

• subject: legen
• verb: undersøker
• object: lungene

Because lungene already includes the meaning of the.

The ending -ne marks definite plural:

• lunger = lungs
• lungene = the lungs

So adding a separate the-word is usually unnecessary here.

Lunge is commonly treated as a feminine noun, so you can say:

• ei lunge
• lunga
• lunger
• lungene

But in Bokmål, many feminine nouns can also be used as masculine, especially in more general or conservative usage:

• en lunge
• lungen

In this sentence, lungene is the plural definite form, and that form works regardless of whether you think of the singular as ei lunge or en lunge.

Because Norwegian verbs do not change form according to the subject or object in the way English sometimes does.

Compare English:

• I examine
• he examines

In Norwegian:

• jeg undersøker
• han undersøker
• de undersøker

The verb form stays the same in the present tense.

Also, the verb does not agree with the object, so lungene being plural does not affect undersøker.

You would say:

En lege undersøker lungene.

Compare:

• Legen undersøker lungene. = The doctor examines the lungs.
• En lege undersøker lungene. = A doctor examines the lungs.

So the difference is:

• en lege = a doctor
• legen = the doctor

That would usually be:

Legen undersøker lunger.

Compare:

• lunger = lungs (indefinite plural)
• lungene = the lungs (definite plural)

So:

• Legen undersøker lunger = The doctor examines lungs
• Legen undersøker lungene = The doctor examines the lungs

In real usage, the definite form may sound more natural if specific lungs are meant, such as a patient’s lungs.

A rough guide is:

UN-der-sur-ker

A few notes:

• ø is a vowel sound that does not exist exactly in English.
• The kj-like sound is not in this word; the k is a normal k.
• Stress is usually on the first syllable: UNDERsøker

If you are learning pronunciation, the hardest part here is usually ø. It is a rounded front vowel, somewhat similar to the vowel in French deux or German schön.

Yes. Norwegian often leaves that kind of information unstated if it is understood from context.

So Legen undersøker lungene could naturally mean that the doctor is examining someone’s lungs, for example a patient’s, even though the sentence literally just says the lungs.

That is similar to English, where The doctor is examining the lungs can also imply a patient’s lungs if the situation is clear.